Sub-pixel nbn detector

ABSTRACT

A method of making a two-dimensional detector array (and of such an array) comprising, for each of a plurality of rows and a plurality of columns of individual detectors, forming an n-doped semiconductor photo absorbing layer, forming a barrier layer comprising one or more of AlSb, AlAsSb, AlGaAsSb, AlSb, AlGaPSb, and HgZnTe, and forming an n-doped semiconductor contact area.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Continuation of pending U.S. application Ser. No.11/939,464, filed on Nov. 13, 2007, which claims priority to and thebenefit of the filing of U.S. Provisional Patent Application Ser. No.60/880,580, entitled “High-Integration Micro-Imager”, filed on Jan. 12,2007, and of U.S. Provisional Patent Application Ser. No. 60/865,793,entitled “SWIR High Integration Micro-Imager Camera”, filed on Nov. 14,2006, and the specifications and claims thereof are incorporated hereinby reference.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not Applicable.

INCORPORATION BY REFERENCE OF MATERIAL SUBMITTED ON A COMPACT DISC

Not Applicable.

COPYRIGHTED MATERIAL

Not Applicable.

BACKGROUND OF THE INVENTION

1. Field of the Invention (Technical Field)

The present invention relates to micro-imaging devices, particularlyinfrared imagers.

2. Description of Related Art

Revolutionary improvements in size and power consumption are required torealize extremely lightweight micro cameras. To that end, the presentinvention is of a highly integrated digital Focal Plane Array thatcombines a fully digital camera sensor engine-on-a-chip with a noveldetector concept that enables high quantum efficiency, diffusion-limiteddetector performance in a small pixel by eliminating the perimeter andsurface leakage currents. Together these innovations will allow themanufacture of low-cost completely uncooled shortwave infrared (SWIR)digital imagers compatible with, for example, unmanned aerial vehicle(UAV) micro-ball gimbaled packaging, with weights under 10 grams and thesensitivity to operate in nightglow environments. The detector conceptsenable higher operating temperature midwave infrared (MWIR) and longwaveinfrared (LWIR) applications as well.

BRIEF SUMMARY OF THE INVENTION

The present invention is of a method of making a two-dimensionaldetector array (and of such an array) comprising, for each of aplurality of rows and a plurality of columns of individual detectors:forming an n-doped semiconductor photo absorbing layer; forming abarrier layer comprising one or more of AlSb, AlAsSb, AlGaAsSb, AlPSb,AlGaPSb, and HgZnTe; and forming an n-doped semiconductor contact area.In the preferred embodiment, forming the absorbing layer comprisesgrading the absorbing layer to create a quasi-electric field drivingminority carriers into the barrier layer, preferably wherein thequasi-electric field has a force on the minority carriers that isstronger than the force of lateral diffusion. Alloys at interfacesbetween the barrier layer and the photo absorbing layer and the contactarea are preferably graded, whereby spikes in minority carrier bandedges are substantially reduced. A diffusion barrier layer between thephoto absorbing layer and a substrate is preferably formed. The contactarea can include a guard n-doped semiconductor, either includingconnecting the guard n-doped semiconductor for each detector via seriesresistors to a guard n-doped semiconductor of one or more adjacentdetectors in the array or wherein the guard n-doped semiconductor ismonolithic and isolated from the active n-doped semiconductor contactarea of each detector. An AlGaSb buffer layer between the n-doped photoabsorbing layer and a substrate is preferably formed, wherein Al contentof the buffer layer is graded. By growing strained AlGaSb superlatticebuffer layers whose average aluminum content increases, the effectivesubstrate lattice constant can be shifted to larger values. Strainedsuperlattice layers can be used to shift lattice constants because thethreading dislocations become bound at the interfaces instead ofcreating defects in the active material.

The present invention is further of a method of making a two-dimensionaldetector array (and of such an array) comprising, for each of aplurality of rows and a plurality of columns of individual detectors:forming an n-doped semiconductor photo absorbing layer; forming anInGaAs barrier layer; and forming an n-doped semiconductor contact area.

Objects, advantages and novel features, and further scope ofapplicability of the present invention will be set forth in part in thedetailed description to follow, taken in conjunction with theaccompanying drawings, and in part will become apparent to those skilledin the art upon examination of the following, or may be learned bypractice of the invention. The objects and advantages of the inventionmay be realized and attained by means of the instrumentalities andcombinations particularly pointed out in the appended claims.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

The accompanying drawings, which are incorporated into and form a partof the specification, illustrate one or more embodiments of the presentinvention and, together with the description, serve to explain theprinciples of the invention. The drawings are only for the purpose ofillustrating one or more preferred embodiments of the invention and arenot to be construed as limiting the invention. In the drawings:

FIG. 1 shows a standard InGaAs detector array and corresponding bandstructure;

FIG. 2 shows an nBn detector array of the invention and correspondingband structure;

FIG. 3 shows band edge vs. lattice constant of common semiconductors,showing the AlGaAsSb barrier alloys selected to match the latticeconstant and valence band offset of InGaAs on InP while presenting ahigh barrier in the conduction band;

FIG. 4 is a graph comparing measured and modeled leakage currentdensity, showing that NBn InAs detectors are diffusion limited above 160K;

FIG. 5 is a graph showing modeled leakage current density for an InGaAsnBn detector and showing that 3 nA/cm.sup.2 requirement can be met iflifetime in active layer exceeds about 1 μsec;

FIG. 6 shows measured signal decay data showing minority carrierlifetime of 950 μsec, which validates that non-fundamentalgeneration-recombination mechanisms are not operative in an InAs nBndetector;

FIG. 7 shows comparable measurements on a large area InSb photodiode,demonstrating lifetime degradation caused by excess g-r and surfacemechanisms;

FIG. 8 is a cross section of an nBn detector according to the invention;note that near-planar architecture and self aligned pixel formationgreatly simplifies the fabrication process and alignment tolerances;

FIG. 9 shows a microlens concentrator and nBn guard according to theinvention; each guard is biased to sweep out unwanted dark current,while maintaining the surface passivation quality of the barrier; eachpixel's guard is electronically isolated by a resistive interconnect tominimize sensitivity to defects;

FIGS. 10 and 11 illustrate the benefits of grading the composition ofthe absorber allow in an nBn junction;

FIG. 12 illustrates band edge problems that can beset an nBn junction;

FIGS. 13 and 14 illustrate the benefits of grading the composition ofthe barrier component of the nBn junction;

FIGS. 15 and 16 illustrate benefits of inclusion of a diffusion barrierand/or an etch stop in an nBn junction;

FIGS. 17-21 show embodiments of uses of biased guard rings to collectunwanted current in nBn structures;

FIG. 22 shows extension of a .about.5 μm cutoff in the n absorber layer;and

FIG. 23 shows the area of bandgap vs. lattice constant suitable forextension to .about.5 μm cutoff wavelengths.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is of a highly integrated digital Focal PlaneArray (FPA) that combines a fully digital camera sensor engine-on-a-chipwith a novel detector concept that enables high quantum efficiency,diffusion-limited detector performance in a small pixel by eliminatingthe perimeter and surface leakage currents. Together these innovationswill allow the manufacture of low-cost completely uncooled shortwaveinfrared (SWIR) digital imagers compatible with, for example, unmannedaerial vehicle (UAV) micro-ball gimbaled packaging, with weights under10 grams and the sensitivity to operate in nightglow environments. Theinvention also concerns technology improvements that make the FPA of theinvention possible, including the improvements to nBn technologydisclosed in U.S. patent application Ser. No. 11/276,962, entitled“Reduced Dark Current Photodetector”, to Shimon Maimon, filed Mar. 19,2006, and published as U.S. Patent Application Publication 2007/0215900.These technology improvements to the nBn detector are applicable to allwavelengths including SWIR, midwave infrared (MWIR) and longwaveinfrared (LWIR) versions of the detector.

The UAV exemplary application of the present invention preferablyprovides for the recognition a 1 m.sup.2 target at a range of 100meters. Target recognition is assumed to occur under worst-caseconditions of a moonless night sky with a 30% target contrast relativeto the background radiance. Using published data on night sky radianceand integrating over a 0.8 μm to 1.7 μm SWIR wave band, targetreflectance is calculated to be 9.8.times.10.sup.9 ph/cm.sup.2-sec-sr.Target recognition implies sufficient signal to noise ratio and spatialresolution. A 1280.times.1024 imaging system with a 40 degree horizontalfield of regard satisfies spatial resolution requirements, placing aminimum of 9 line pairs on a 1 m.times.1 m target at 100 meters. Asignal to noise ratio of 3 was selected as a minimum for the purpose ofthis analysis.

A radiometric model was generated which uses measured and modeleddetector data and ROIC performance predictions based on previousdesigns. At an operating temperature of 27 C (300 K), target recognitiongoals are met under worst case night glow radiance. The camera system ofthe invention preferably comprises a manufacturable f/#1.4 opticalsystem coupled to a low noise focal plane array (FPA). Significantlyhigher system operating temperatures are possible during daytimeoperation where radiance and signal are substantial. A summary ofoptical system, detector, and ROIC parameters required to meetperformance goals are tabulated in Table 1. Note that a detector fillfactor is used, assuming a light concentrator such as a microlensreduces the active pixel area and hence the dark current.

TABLE 1 Preferred System Parameters Parameter Requirement Units OpticalSystem f/# 1.4 Optical Path Transmission 0.6 Detector Pitch 15 umDetector Leakage Current Density 3.00E+09 A/cm{circumflex over ( )}2Detector Dynamic Resistance 9.50E+13 Ohms Detector Spectal Sensitivity0.8-1.7 um Detector Quantum Efficiency 0.9 e-/ph Detector Physical FillFactor 0.2 Integration Time 16.6 mSec ROIC Total Noise 20 e-

InGaAs detectors lattice matched to InP provide uncooled operation withcutoff wavelengths of 1.7 μm. A review of the literature shows that theleakage current density of large area InGaAs detectors operating at roomtemperature is on the order of a few nA/cm.sup.2. However, since smallpixel sizes are required for imaging arrays, edge and surface effectsbecome dominant. As the perimeter-to-area ratio increases, the darkcurrent density can increase by a factor of 10 or more. For example,recent state-of-the-art room temperature data on a 25 μm pixel shows adark current of 130 fA at 0.3V reverse bias or equivalently 19nA/cm.sup.2. The sources of these leakage currents are shown in FIG. 3,where heterojunction interfaces are shown as abrupt vs. graded forclarity. Most InGaAs detectors use an InP cap layer to reduce thesurface leakage, however the high fields across the p-n junction remainsensitive to surface interface traps. Similar effects can occur at theheterointerface across the lateral p-n junction, as well as the midgapgeneration currents in the depletion region within the narrow gapInGaAs.

The nBn detector structure of the invention is shown in FIG. 2, wherethe standard InGaAs absorber now has a thin barrier layer and contactlayer that replaces the p-n junction. Photo-generated minority carriers(holes) are collected while the barrier blocks majority carrier(electron) flow. The most notable difference is the lack of a p-njunction and the associated high fields within the narrow gap materials.The high fields across the p-n junction are the source of leakagecurrents in standard detectors. Instead, a barrier layer has been grownon the surface of the InGaAs with the unique property of presenting ahigh barrier to carrier flow in the conduction band, but with negligiblebarrier in the valence band. This enables a bias to be placed across thedevice without majority carrier current flow, while allowing thecollection of photo-generated minority carriers by diffusion. Theundoped wide-bandgap barrier acts simultaneously as surface passivation,contact isolation and minority carrier collector. Thus, there are nohigh lateral fields in the device, eliminating the perimeter leakageeffects. Modeling as discussed later indicates the large areaperformance of 3 nA/cm.sup.2 is achievable with a small pixel nBndetector.

Device fabrication is simple as well, making large arrays of very smallpixels possible. The barrier layer itself is relatively thin, typicallytwo thousand angstroms or less, and thus it can support latitude forstrain in the barrier growth without detriment to device performance.The bandgap engineering of the nBn device requires knowledge of theconduction band and valence band offsets. The band offsets for therelevant materials vs. lattice constant is shown in FIG. 3. The dualvertical points identify the conduction band energy for a material atthe upper point and the valance band energy at the lower point. Thebandgap, conduction and valence band edges are shown forIn.sub.0.53Ga.sub.0.47As lattice matched with InP in red. Circled arethe band edges for AlGaAs and for AlGaSb. It is apparent that a latticematch to InP can be achieved with an AlGaAsSb quaternary while producinga zero valance band offset as shown in the inset band diagram.

In order to reduce the detector dark current further to meet thestringent nightglow requirements, a sub-pixel active region can be usedto reduce the dark current. This can be realized with the simpleself-aligned junction processing for an nBn detector since there are notdepletion zones nor diffusion or implantation processing. In the SWIRexample, a standard InGaAs absorber is grown on InP, then the thinBarrier and top n contact layers complete the detector. As shown in FIG.2, ohmic metal is deposited on the n contact layer, then selectiveetches are used to isolate the n contact using the contact metal as aself aligned mask. Pixel sizes below 4 μm are practically realized. If a4 μm active is formed producing a 6 μm diameter effective active area ina 15 μm pixel, the dark current is reduced by 7.times.

Microlens arrays or other light concentrators are required to takeadvantage of the sub-pixel active region. It is preferred to use a closeproximity filter that enables placement of filters or microlens arraysto better than 2 microns true position in x, y and z across the focalplane. This microlens technique, commonly used in visible cameras,provides an additional reduction of the dark current so that thenightglow sensitivity requirement can be achieved with acceptable signalto noise ratio.

The above system analysis shows that the detector leakage currentdensity must preferably be less than 3 nA/cm.sup.2, including 1/f noise,and quantum efficiency must be greater than 90% if a 15 μm pixel-arrayis to resolve a 1 m.sup.2 target under the most stressing conditions ofa moonless, rural night sky at an ambient temperature of 300 K. Marginto these requirements is provided by incorporating a microlens array tothe back surface of the FPA such that the incident IR signal onto a 15μM pixel is concentrated into a 5 μm active area. The followingdiscussion demonstrates that these conditions are met with nBn-basedfocal planes.

Routinely achieving this leakage current density is the most challengingproblem for the detector manufacturer because all but the mostfundamental leakage mechanisms must be eliminated, or at least reducedto the point that they do not add significantly to the detector'sleakage current. As shown above in FIG. 1, this is very difficult forconventional photovoltaic devices for two reasons. First, the highelectric fields in the depletion region of the p-n junction causes anycarrier generated in that region to be swept to and collected by thecontacts, leading to what is commonly referred to as G-R current.Crystalline imperfections and impurities associated within this region,whether they are the result of the junction formation process orresidual defects within the material, can cause the generation of excesscarriers whose flow can easily become the dominant leakage currentmechanism. This is especially true in the region where the depletionregion intersects the diode's surface because the termination of thedetector's crystalline structure makes it even more difficult to avoiddefects which give rise to this current. The surface of the neutralabsorbing material is the second source of excess leakage current, againbecause of the difficulty in terminating the material's crystallinestructure without creating defects and contamination, both of which aresources of excess carrier generation which increase leakage current.U.S. patent application Ser. No. 11/276,962 discloses a new type ofsemiconductor device, nBn detector, described above in FIG. 2, whosebasic design eliminates both of these non-fundamental currentmechanisms, leaving only diffusion current from the active absorbingmaterial as the limiting mechanism.

Tests on InAs nBn devices, with a cutoff wavelength of 3.6 μm, havevalidated this device concept. These measured data are compared tomodeled diffusion-limited performance in FIG. 4, showing they arelimited only by diffusion current for temperatures above 160 K.

The same model is easily extrapolated to In.sub.xGa.sub.1-xAs, wherex=0.53 to meet the 1.7 μm cutoff wavelength required by themicro-platform missions. The results shown in FIG. 5, as plots ofleakage current density versus detector temperature, demonstrate thatleakage current density of less than 3 nA/cm.sup.2 can be obtained atand below 300 K. The only parameters needed to determine this diffusioncurrent are the thickness and doping concentration of the absorbingdetector material and the lifetime of the minority carrier hole. Allother parameters, such as intrinsic carrier concentration and minoritycarrier mobility, are fixed material properties. The modeled curves inFIG. 5 assume an absorbing layer thickness of 2 μm, doping concentrationof 2.times.10.sup.16 cm.sup.−3 and hole mobility of 500 cm.sup.2N-sec.The model also assumed a minority carrier lifetime of either 1 or 10μsec, either of which are reasonable for high quality InGaAs material.The longer 10 μsec material provides about 10 K higher operatingtemperature, but is not required to meet the requirements.

A second validation of the nBn concept comes from an analysis of thechopped blackbody signal measured on InAs detectors. The decay of thesignal amplitude, shown in FIG. 6, is fit to a single exponential decaywith a time constant of 950 μsec. This very long lifetime can only beachieved if recombination mechanisms associated with material defectsand surfaces are not present. By way of contrast, a similar analysis ofa large-area InSb photodiode measured under the same conditions in FIG.7 show that the signal's decay is much faster than the nBn detector, andis limited by the rate at which the chopper closes the blackbodyaperture. Other measurements on InSb diodes show its effective lifetimeto be less than 10 nsec, or about 2 orders of magnitude less than thenominal 1 μsec lifetime of the bulk InSb material.

The final conclusion is that a leakage current density of 3 nA/cm.sup.2can be achieved in very small InGaAs nBn devices because the activeportion of the device is “buried” within wide bandgap materials andwithout reverse bias junction fields so that only by the mostfundamental diffusion current mechanism is present.

Low frequency noise becomes a major concern, especially when requiringextremely low detector noise in a staring-format FPA. The observationmade when testing the InAs nBn devices was that no 1/f noise was seendown to 1 Hz, even when the devices were biased to 2.0 V. Thisobservation validates the claims that the nBn design eliminates excessleakage currents because 1/f noise is almost always associated withexcess, i.e., non-diffusion, leakage current, especially when thatcurrent originates from tunneling or shunt resistance mechanisms.

Chopped blackbody measurements showed the quantum efficiency of theuncoated InAs devices to be 60%, a value that is limited only byreflection at the vacuum/InAs interface. Adding a simple 3 or 5 layerantireflection coating to the back surface of the detector substratewill easily reduce this loss to below 10%, thereby enabling the 90%quantum efficiency needed to meet the system flow down.

The simplicity of the nBn structure makes it ideally suited for opticalconcentration. As shown in FIG. 8, the n absorber layer is continuousover the entire device as is the nominally 2000 .ANG. thick Barrierlayer except at its edges where ground contact is made to the absorberlayer. Also, the n contact and metal layers are each less than 1000.ANG. thick, so the device's surface is essentially planar. The processto fabricate this device is also quite simple, comprising the followingsteps:

1. Mask and deposit contact metal to top n layer

2. Lift-off metal and etch to remove n layer between contacts and formindividual pixels

3. Mask and etch to remove n and Barrier layers to expose absorber layer

4. Deposit ground metal and lift-off

5. Deposit Si.sub.3N.sub.4 overglass over entire wafer

6. Mask and etch openings to contacts and grounds

7. Mask for In bumps

8. Deposit In and lift-off

9. Mask and etch saw lanes

10. Dice, clean and demount

The only critical mask step is the formation of the Array N ohmic—allother steps are self aligned or non critical. Since the structure isessentially planar, dimensions on the order of 1 μm are straightforward.Also, since the device is protected by a Si.sub.3N.sub.4 overglass, theindium bump can extend over the edge of the 5 μm diameter pixel,eliminating alignment and lithography concerns.

The microlens shown schematically in FIG. 9 will concentrate the lightinto the 5 μm diameter active area, thereby reducing the volume ofmaterial which generates diffusion current by a factor of15.sup.2/(.pi.2.5.sup.2)=11.5, but this leaves the problem of what to dowith the diffusion current from the remainder of the 15 μm unit cell. Aninnovative solution to this problem is shown in FIG. 8 where large guardcontacts that fill the areas not taken by the active areas are connectedto their neighbors with four thin metal straps. The guards are then tiedto a reverse bias supply around the perimeter of the array. The metalstrips solve the problem that occurs when a short between one of theguards and the absorber layer occurs somewhere on the array, becausethey provide a spreading resistance which confines the resulting voltagedrop to the immediate vicinity of the short. While this can create asmall defect cluster, it eliminates the yield loss otherwise caused by ashorted guard.

Calculations of the optical performance including the microlens of theinvention are show that both on axis and off axis cases support thespectral and optical transmission requirements and achieve the desiredlight concentration within a 5 μm diameter. The optimal sub-pixel activeregion location shifts as the position moves from center to edge, thiscan be accommodated for in the detector layout.

The present invention also comprises certain improvements to the basenBn infra-red detector. The improvements result in improved modulationtransfer function, reduced dark current and designs that are more robustto manufacturing tolerances associated with the epitaxial growth of thesemiconductor crystal. Alloy grading of the absorber is preferred, whichprovides an electrochemical field that will drive minority carriers tobe collected in the contact instead of laterally diffusing to anadjacent pixel, Detector structures with sub-pitch pixels with biasedguard rings reduce the dark current which is important for elevatedoperating temperature. The use of a hole diffusion barrier at thesubstrate eliminates excess dark currents while graded interfaces on theBarrier avoids heterointerface notches that can increase bias voltagesdue to thermionic emission limits. Specific embodiments are disclosedthat enable the extension of the MWIR applications to SWIR and out tothe long wavelength side (5 μm) of MWIR applications.

The improvements relate to six primary areas: (1) The photo generatedminority carriers generated in the absorber diffuse to the barrier;lateral diffusion to adjacent pixels degrades the Modulation TransferFunction, as is observed in InSb MWIR focal plane arrays; (2) Non-idealgrowth of the barrier alloys can lead to spikes in the minority carrierband edge, requiring excess voltage to turn on minority currentcollection; (3) The base nBn detector shows no minority carrier barrierbetween the absorber and the substrate, leading to excess dark currentsfrom thermally generated carriers in the substrate; (4) The nBnstructure does not allow mesa isolation of small pixels; making a small“detector within a pixel” allows a .about.10.times. reduction in darkcurrent, enabling even higher operating temperatures; (5) Cutoffwavelengths of nBn detectors grown on GaSb is .about.4.5 μm; extendingthe cutoff to .about.5 μm enables application to two-color threatwarning applications; and (6) InGaAs is a mature 1.7 μm cutoff IRdetector for SWIR applications; materials are disclosed that enable nBnstructures on InGaAs.

As to area (1), by grading the composition of the absorber alloy, thechemical potential creates a quasi-electric field that drives theminority carriers into the barrier. This quasi-field is much strongerthan diffusion, so the lateral diffusion is dramatically reduced. Thisgraded bandgap effect has been used on heterojunction bipolartransistors and HgCdTe IR p-n junction detector but has not beenproposed or claimed for use in an nBn junction IR detector. See FIGS. 10and 11.

As to area (2), consider the conditions shown in FIG. 12. The spikecannot be eliminated with increasing bias voltage. The relative bandoffsets of the barrier and absorber change with temperature as well,complicating the optimal design. The abrupt interface case can lead to anotch in the band edge which traps the minority carriers. A localquasi-Fermi level rises as the carrier density, p, increases until thethermionic, tunneling and recombination currents equal the incomingdiffusion current. Carriers that recombine do not generate signalcurrent at the contacts, degrading detector performance and requiringincreased bias voltage to collect the current. Referring to FIGS. 13 and14, by grading the composition of the barrier to absorber and contactinterfaces, the spikes at the heterointerfaces are eliminated. A muchlower electric field can be applied to bias the collection of carriersin this non-ideal (and likely) case, making the design more robust tovariations of alloys in the growth process.

As to area (3), and referring to FIGS. 15 and 16, introducing a “nonnBn” band structure that blocks minority carriers from the substratefrom reaching the absorber, and visa versa, both avoid excess darkcurrent and ensures photo-generated carriers do not recombine at thesubstrate interface. The diffusion barrier can act as or be combinedwith an etch stop to enable backside substrate removal via a chemicaletch for the case of non-transparent substrates. GaSb, for example istransparent in the MWIR while InAs is absorbing.

As to area (4), and referring to FIGS. 17-21, making a small active areawithin each pixel will collect current from the region around it unlessa biased guard ring collects this unwanted current. The light must befocused down into the region under the active area with a lightconcentrator. Several versions are shown, from a common guard to a guardtied with series resistors to a guard per pixel with two interconnectsper pixel, with the latter providing the highest tolerance to shortingdefects in the guard barrier layer at the sake of process complexity.

As to area (5), and referring to FIG. 23, the bandgap (and cutoffwavelength) vs. lattice constant map for the nBn materials is shown.Binary substrates such as InAs and GaSb are candidates for growing thenBn detectors with MWIR sensitivity (3<.lamda.<5 mm). However, thelattice matched alloys to the GaSb cutoff at 4.5 μm, and it is desirableto reach a 4.8 μm or longer for threat warning applications. By growingstrained AlGaSb superlattice buffer layers whose average aluminumcontent increases as shown in FIG. 22, the effective substrate latticeconstant can be shifted to larger values. Pure AlSb, while chemicallyreactive with oxygen, has a lattice constant of 6.13 Angstroms whichmatches InAsSb alloys with a cutoff wavelength equal to InSb, .about.5.3mm. Strained superlattice layers can be used to shift lattice constantsbecause the threading dislocations become bound at the interfacesinstead of creating defects in the active material.

As to area (6), FIG. 3 shows alloys that can be used to create an nBnstructure on the relatively mature InGaAs material system. The nBnstructure, by eliminating excess leakage currents, enables higheroperating temperature uncooled SWIR arrays for a broad range ofapplications including IR imaging.

Although the invention has been described in detail with particularreference to these preferred embodiments, other embodiments can achievethe same results. Variations and modifications of the present inventionwill be obvious to those skilled in the art and it is intended to coverin the appended claims all such modifications and equivalents. Theentire disclosures of all references, applications, patents, andpublications cited above are hereby incorporated by reference.

1-25. (canceled)
 26. A photodetector comprising: a photo absorbing layerhaving a predetermined doping type, a contact layer having thepredetermined doping type, and a barrier layer disposed between saidphoto absorbing and contact layers, said barrier layer being configuredto prevent majority carrier transport; where the barrier layer is gradedwith a first grading at a first interface area between said barrier andsaid contact layer and graded with a second grading at a secondinterface area between said barrier and said photo absorbing layer suchthat the first grading creates a graded transition from the minoritycarrier band edge of the barrier layer to the minority carrier band edgeof the contact layer; and the second grading creates a graded transitionfrom the minority carrier band edge of the photo absorbing layer to theminority carrier band edge of the barrier layer.
 27. The photodetectorof claim 26, where the first grading is configured to reduce minoritycarrier recombination by reducing the size of a carrier trap at thefirst interface.
 28. The photodetector of claim 26, where the secondgrading is configured to enable detector operation at a reduced biasvoltage level by reducing minority carrier recombination rates at thesecond interface.
 29. The photodetector of claim 26, where the contactlayer includes a plurality of contact regions, each contact regionrepresenting a pixel.
 30. The photodetector of claim 26, where thepredetermined doping type is n-type.
 31. The photodetector of claim 26,where the barrier layer is un-doped.
 32. The photodetector of claim 29,where at least one contact region includes a guard area and an activearea, said guard and active areas being physically distinct.
 33. Thephotodetector of claim 32, where a guard area from a first contactregion is electrically connected to a guard area from a second contactregion.
 34. The photodetector of claim 26, where photo absorbing layerincludes an absorber alloy and where the absorber alloy is graded. 35.The photodetector of claim 26, where the photo absorbing layer includesa chemical potential in the alloy to drive minority carriers from thephoto absorbing layer into the barrier layer.
 36. The photodetector ofclaim 35, where the chemical potential reduces lateral diffusion ofminority carriers through the photo absorbing layer.
 37. Thephotodetector of claim 35, where the chemical potential is accomplishedby grading the composition of the absorber alloy.
 38. The photodetectorof claim 26, where the graded absorber alloy and the second gradingcreate the second interface area.
 39. The photodetector of claim 35, thedetector further comprising a substrate layer disposed on the photoabsorbing layer such that the photo absorbing layer is between saidsubstrate layer and said barrier layer; and a diffusion barrier portiondisposed between said photo absorbing layer and said substrate, saiddiffusion barrier portion preventing flow of minority carriers generatedoutside of the photo absorbing layer between said substrate layer andsaid photo absorbing layer.
 40. The photodetector of claim 26, thedetector further comprising a substrate layer disposed on the photoabsorbing layer such that the photo absorbing layer is between saidsubstrate layer and said barrier layer; and a diffusion barrier portiondisposed between said photo absorbing layer and said substrate, saiddiffusion barrier portion preventing flow of minority carriers generatedoutside of the photo absorbing layer between said substrate layer andsaid photo absorbing layer.
 41. The detector of claim 40, where thediffusion barrier is includes at least a first and a second strainedsuperlattice buffer layer, the first and second strained superlatticebuffer layers having different alloy compositions.
 42. The detector ofclaim 41, wherein the diffusion barrier includes layers of three or moredifferent alloy compositions.
 43. A photodetector comprising: a photoabsorbing layer having a predetermined doping type, a contact layerhaving the predetermined doping type, and a barrier layer disposedbetween said photo absorbing and contact layers, said barrier layerbeing configured to prevent majority carrier transport; where thebarrier layer is graded with a first grading at a first interface areabetween said barrier and said contact layer such that the first gradingcreates a graded transition from the minority carrier band edge of thebarrier layer to the minority carrier band edge of the contact layer.44. The detector of claim 43, where the barrier layer is graded with asecond grading at a second interface area between said barrier and saidphoto absorbing layer such that the second grading creates a gradedtransition from the minority carrier band edge of the photo absorbinglayer to the minority carrier band edge of the barrier layer.
 45. Thephotodetector of claim 43, where photo absorbing layer includes anabsorber alloy and where the absorber alloy includes a chemicalpotential in the alloy to drive minority carriers from the photoabsorbing layer into the barrier layer.
 46. The photodetector of claim43, where the first grading is configured to reduce minority carrierrecombination by reducing the size of a carrier trap at the firstinterface.
 47. The photodetector of claim 45, where the chemicalpotential reduces lateral diffusion of minority carriers through thephoto absorbing layer.
 48. The photodetector of claim 43, where thephoto absorbing layer is graded.
 49. The photodetector of claim 44,where photo absorbing layer includes an absorber alloy and where thecomposition absorber alloy is graded in a way that creates a chemicalpotential in the alloy to drive minority carriers from the photoabsorbing layer into the barrier layer; and where the graded absorberalloy and the second grading create the second interface.
 50. Aphotodetector comprising: a photo absorbing layer having a predetermineddoping type, a contact layer having the predetermined doping type, and abarrier layer disposed between said photo absorbing and contact layers,said barrier layer being configured to prevent majority carriertransport; where the barrier layer is graded with a grading at aninterface area between said barrier and said photo absorbing layer suchthat the grading creates a graded transition from the minority carrierband edge of the barrier layer to the minority carrier band edge of thephoto absorbing layer.
 51. The photodetector of claim 50, where photoabsorbing layer includes an absorber alloy and where the composition ofthe absorber alloy is graded in a way that creates a chemical potentialin the alloy to drive minority carriers from the photo absorbing layerinto the barrier layer.
 52. The photodetector of claim 51, where thechemical potential reduces lateral diffusion of minority carriersthrough the photo absorbing layer.
 53. The photodetector of claim 51,where the graded absorber alloy is graded by varying Al content in theabsorber alloy.
 54. The photodetector of claim 51, where the gradedabsorber alloy and the second grading create the interface.
 55. Aphotodetector comprising: a photo absorbing layer having a predetermineddoping type, a contact layer having the predetermined doping type, and abarrier layer disposed between said photo absorbing and contact layers,said barrier layer being configured to prevent majority carriertransport; where the barrier layer is graded with a grading that createsa graded transition from a minority carrier band edge of the barrierlayer to a minority carrier band edge of the photo absorbing layer. 56.The photodetector of claim 55, the detector further comprising asubstrate layer disposed on the photo absorbing layer such that thephoto absorbing layer is between said substrate layer and said barrierlayer; and a diffusion barrier portion disposed between said photoabsorbing layer and said substrate, said diffusion barrier portionpreventing flow of minority carriers generated outside of the photoabsorbing layer between said substrate layer and said photo absorbinglayer.
 57. The detector of claim 56, where the diffusion barrier isincludes at least a first and a second strained superlattice bufferlayer, the first and second strained superlattice buffer layers havingdifferent alloy compositions.
 58. The detector of claim 57, wherein thediffusion barrier includes layers of three or more different alloycompositions.
 59. The detector of claim 56, where the diffusion barrierincludes aluminum and where the aluminum content of the diffusionbarrier is graded.
 60. The detector of claim 59, where the Al content ofthe diffusion barrier is graded by including, in said buffer layer, atleast a first and a second strained superlattice buffer layer, the firstand second strained superlattice buffer layers having different Alcontent levels.
 61. The photodetector of claim 55, where photo absorbinglayer includes an absorber alloy and where the absorber alloy includes achemical potential in the alloy to drive minority carriers from thephoto absorbing layer into the barrier layer.
 62. The photodetector ofclaim 55, where the first grading is configured to reduce minoritycarrier recombination by reducing the size of a carrier trap at thefirst interface.
 63. The photodetector of claim 61, where the chemicalpotential reduces lateral diffusion of minority carriers through thephoto absorbing layer.
 64. The photodetector of claim 55, where thechemical potential is created by varying Al content in the absorberalloy.
 65. The photodetector of claim 55, where the photo absorbinglayer is graded.
 66. A photodetector comprising: a photo absorbing layerhaving a predetermined doping type, a contact layer having thepredetermined doping type, and a barrier layer disposed between saidphoto absorbing and contact layers, said barrier layer being configuredto prevent majority carrier transport; where the barrier layer is gradedwith a first grading that creates a graded transition from the minoritycarrier band edge of the barrier layer to the minority carrier band edgeof the contact layer.
 67. The detector of claim 66, where the barrierlayer is graded with a second grading that creates a graded transitionfrom the minority carrier band edge of the photo absorbing layer to theminority carrier band edge of the barrier layer.
 68. A photodetectorcomprising: a photo absorbing layer having a predetermined doping type,a contact layer having the predetermined doping type, and a barrierlayer disposed between said photo absorbing and contact layers, saidbarrier layer being configured to prevent majority carrier transport;where the barrier layer is graded with a grading that creates a gradedtransition from the minority carrier band edge of the barrier layer tothe minority carrier band edge of the contact layer and also creates agraded transition from the minority carrier band edge of the photoabsorbing layer to the minority carrier band edge of the barrier layer.69. The photodetector of claim 68, where the contact layer includes aplurality of contact regions, each contact region representing a pixel.70. The photodetector of claim 68, where the predetermined doping typeis n-type.
 71. The photodetector of claim 68, where the barrier layer isun-doped.
 72. A detector comprising: an absorption layer for generatingelectron-hole pairs in response to absorbed photons, the absorptionlayer comprising a semiconductor having majority carriers and minoritycarriers; a barrier layer comprising a semiconductor adjacent to theabsorption layer to provide a barrier to the majority carriers, thebarrier layer comprising a concentration gradient; and a contact layercomprising a semiconductor adjacent to the barrier layer.
 73. Thedetector as set forth in claim 72, where the absorption layer and thecontact layer are n-type semiconductors so that the majority carriersare electrons and the minority carriers are holes.
 74. The detector asset forth in claim 72, the absorption layer, the barrier layer, and thecontact layer each having a band for conduction of the minoritycarriers, the concentration gradient such that the band is alignedacross the barrier layer and the contact layer.
 75. The photodetector ofclaim 72, where photo absorption layer includes an absorber alloy andwhere the absorber alloy includes a chemical potential in the alloy todrive minority carriers from the absorption layer into the barrierlayer.
 76. The photodetector of claim 72, where the concentrationgradient is configured to reduce minority carrier recombination byreducing the size of a carrier trap at an interface of the barrier andcontact layers.
 77. The photodetector of claim 75, where the chemicalpotential reduces lateral diffusion of minority carriers through theabsorption layer.
 78. The photodetector of claim 75, where the chemicalpotential is created by varying Al content in the absorber alloy. 79.The photodetector of claim 72, where the absorption layer is graded. 80.The photodetector of claim 72, the detector further comprising asubstrate layer disposed on the absorption layer such that theabsorption layer is between said substrate layer and said barrier layer;and a diffusion barrier portion disposed between said absorption layerand said substrate, said diffusion barrier portion preventing flow ofminority carriers generated outside of the absorption layer between saidsubstrate layer and said absorption layer.
 81. The detector of claim 80,where the diffusion barrier is includes at least a first and a secondstrained superlattice buffer layer, the first and second strainedsuperlattice buffer layers having different alloy compositions.
 82. Thedetector of claim 81, wherein the diffusion barrier includes layers ofthree or more different alloy compositions.
 83. The detector of claim80, where the diffusion barrier includes aluminum and where the aluminumcontent of the diffusion barrier is graded.
 84. The detector of claim83, where the Al content of the diffusion barrier is graded byincluding, in said buffer layer, at least a first and a second strainedsuperlattice buffer layer, the first and second strained superlatticebuffer layers having different Al content levels.
 85. The detector ofclaim 72, where the concentration gradient is present at least one of aninterface of the barrier and absorption layers and an interface of thebarrier and contact layers.
 86. The detector of claim 85, where theconcentration gradient is present at both interfaces.
 87. A detectorcomprising: an absorption layer for generating electron-hole pairs inresponse to absorbed photons, the absorption layer comprising asemiconductor having majority carriers and minority carriers; a barrierlayer comprising a semiconductor adjacent to the absorption layer toprovide a barrier to the majority carriers; and a contact layercomprising a semiconductor adjacent to the barrier layer, where thephoto absorbing layer includes a chemical potential in the alloy todrive minority carriers from the photo absorbing layer into the barrierlayer.
 88. The photodetector of claim 87, where the chemical potentialis accomplished by grading the composition of the absorber alloy.
 89. Adetector comprising: an absorption layer for generating electron-holepairs in response to absorbed photons, the absorption layer comprising asemiconductor having majority carriers and minority carriers; a barrierlayer comprising a semiconductor adjacent to the absorption layer toprovide a barrier to the majority carriers; and a contact layercomprising a semiconductor adjacent to the barrier layer, where thephoto absorbing layer includes an absorber alloy and where the absorberalloy is graded.
 90. The photodetector of claim 89, where the gradedabsorber alloy is graded by varying Al content in the absorber alloy.91. A detector comprising: an absorption layer for generatingelectron-hole pairs in response to absorbed photons, the absorptionlayer comprising a semiconductor having majority carriers and minoritycarriers; a barrier layer comprising a semiconductor adjacent to theabsorption layer to provide a barrier to the majority carriers; acontact layer comprising a semiconductor adjacent to the barrier layer;a substrate layer disposed on the photo absorbing layer such that thephoto absorbing layer is between said substrate layer and said barrierlayer; and a diffusion barrier portion disposed between said photoabsorbing layer and said substrate, said diffusion barrier portionpreventing flow of minority carriers generated outside of the photoabsorbing layer between said substrate layer and said photo absorbinglayer; where said diffusion barrier portion is graded.
 92. The detectorof claim 91, where the diffusion barrier is graded by including, in saidbuffer layer, at least a first and a second strained superlattice bufferlayer, the first and second strained superlattice buffer layers havingdifferent alloy compositions.
 93. The detector of claim 92, wherein thediffusion barrier includes layers of three or more different alloycompositions.
 94. The detector of claim 92, where the diffusion barrierincludes aluminum and where the aluminum content of the diffusionbarrier is graded.
 95. The detector of claim 94, where the Al content ofthe diffusion barrier is graded varying the Al composition of thestrained superlattice layers.